Neutral gray, fixed tint sunglasses have been marketed for may years. Bausch and Lomb, Rochester, N.Y. has marketed a glass G15 since the 1940s and Corning Incorporated, Corning, N.Y. has marketed Code 8364, included within U.S. Pat. No. 2,688,561 (Armistead), since the 1950s. Corning has also manufactured a glass having a composition similar to that of G-15 under Code 8015. Analyses of 8015 and 8364, reported in terms of weight percent on the oxide basis, are recorded below.
______________________________________ 8015 8364 ______________________________________ SiO.sub.2 68.41 65.4 Al.sub.2 O.sub.3 0.51 2.0 Na.sub.2 O 8.81 7.25 K.sub.2 O 9.71 10.6 ZnO 6.76 13.52 Fe.sub.2 O.sub.3 5.54 1.7 Co.sub.3 O.sub.4 0.021 -- NiO 0.126 0.21 As.sub.2 O.sub.3 0.111 -- TiO.sub.2 -- 0.3 ______________________________________
Both of those glasses appear neutral gray to the eye when viewed alone. When they are examined side-by-side, however, Code 8015 glass appears greenish gay compared to Code 8364 glass. On the other hand, Code 8364 glass assumes a brownish gray hue when viewed next to a glass of a more neutral gray; i.e., a glass having a chromaticity closer to the illuminant.
As can be readily appreciated, the closer the tint of a glass approaches the neutral gray of the illuminant, the better each color retains its relationship with the other colors of the spectrum. Stated in another way, the color balance is maintained; e.g., the sky exhibits a truer blue hue, not bluish green as viewed through glass Code 8015.
Accordingly, the first vital objective of the present invention was to develop a glass suitable for use as a sunglass exhibiting a fixed tint gray coloration which closely approaches the illuminant, and which limits the transmittance of ultraviolet radiation at a wavelength of 380 nm to no more than 1%.
As was observed in Ser. No. 07/982,562, the optical and ophthalmic laboratories have been under increasing pressure to reduce the level of zinc released in the effluent flowing from their finishing operations. Therefore, the second vital objective of the instant invention was to design glass compositions for use as sunglasses displaying the desired neutral gray tint and limitation of ultraviolet radiation transmission, and which would have a very low level of zinc; preferably no substantial quantity of zinc is purposefully added to the compositions.
Also, as was explained in Ser. No. 07/982,562, the presence of CaO in a glass composition appears to block or otherwise restrict an ion exchange reaction taking place between K+ and Na+ ions. As a result, the surface compression layer developed during the ion exchange reaction is quite shallow such that, whereas it imparts a very sizeable immediate improvement in mechanical strength, the glass is subject to a significant decrease in strength when exposed to surface abuse commonly encountered in everyday use. Consequently, it has been considered most desirable to avoid any substantial concentration of CaO in glasses which are scheduled to be chemically strengthened and, hence, a subsidiary objective of the subject invention was to devise a range of preferred glasses satisfying the above objectives wherein the compositions thereof contain CaO in very low amounts and can be essentially free from CaO.